**Core Concept**
The process of active reabsorption of sodium ions in the kidney is crucial for maintaining electrolyte balance and fluid status in the body. This process involves the coordinated action of various transport proteins and ion channels in the renal epithelial cells.

**Why the Correct Answer is Right**
Active reabsorption of sodium ions primarily occurs in the proximal convoluted tubule (PCT), where it is mediated by the sodium-potassium pump (Na+/K+-ATPase) and the sodium-glucose cotransporter 2 (SGLT2). The sodium-potassium pump uses ATP to pump sodium ions out of the cell and into the interstitium, while SGLT2 cotransports sodium ions with glucose molecules into the interstitium. This process is essential for reabsorbing approximately 60-70% of filtered sodium ions back into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** The loop of Henle is primarily involved in passive reabsorption of ions and water through the countercurrent multiplier mechanism, but it is not the primary site for active reabsorption of sodium ions.
**Option B:** The distal convoluted tubule (DCT) is involved in fine-tuning sodium reabsorption and secretion, but it is not the primary site for active reabsorption of sodium ions.
**Option C:** The collecting duct is primarily involved in regulating potassium and hydrogen ion secretion, as well as water reabsorption under the influence of antidiuretic hormone (ADH), but it is not the primary site for active reabsorption of sodium ions.

**Clinical Pearl / High-Yield Fact**
The kidneys reabsorb approximately 180-200 mEq of sodium ions per day, which is equivalent to about 1-1.5 liters of water. This process is essential for maintaining blood pressure and electrolyte balance in the body.

**Correct Answer:** C. Proximal convoluted tubule.